The present invention relates to the field of semiconductor devices and semiconductor processing. More particularly, the present invention relates to methods of suppressing or eliminating transient enhanced diffusion (TED) of ion implanted dopant impurities in semiconductor substrates. Even more particularly, the present invention relates to reduction or elimination of TED of ion implanted p- or n-type dopant impurities in silicon semiconductor substrates.
The escalating requirements for high density and performance associated with ultra-large-scale integration (ULSI) semiconductor devices require design features of 0.18 xcexcm and below, such as 0.15 xcexcm below, increased transistor and circuit speeds, high reliability, and increased manufacturing yield and throughput for economic competitiveness. The reduction of design features to 0.18 xcexcm and below challenges the limitations of conventional semiconductor manufacturing techniques.
The abovementioned continuing trend toward greater microminiaturization of semiconductor devices has engendered a need for reduced junction depths of integrated circuit components, e.g., transistors, to between about 70-100 nm. A problem confronting the achievement of such shallow junction depths is transient enhanced diffusion (xe2x80x9cTEDxe2x80x9d), i.e., redistribution of implanted dopant impurities during thermal annealing following ion implantation of the dopant.
In semiconductor device fabrication, dopant containing impurities, i.e., n- and/or p-type dopant impurities, are introduced into the semiconductor material to alter its electrical conductivity characteristics. A preferred technique for use in high-density integration semiconductor devices is ion implantation. During ion implantation, a beam containing ions is directed at the surface of the semiconductor substrate material (i.e., target). As the ions enter the substrate material, they collide with the atoms constituting the target material and come to rest at an average depth within the substrate. The average depth at which the implanted ions are distributed varies with the implant energy, size (i.e., radius) of the implanted species, and the target material. For a given target material and ion species, higher implantation energy generally corresponds to a deeper penetration of the ions into the material. The implantation dose (i.e., total number of ions entering the target material) is controlled by monitoring the ion current during implantation as well as the duration thereof.
However, ion implantation incurs undesirable side effects. For example, during implantation the impinging ions displace target atoms, thereby creating damage in the crystal lattice structure of the target. Such damage typically is in the form of vacancies (i.e., holes or vacant lattice sites) and interstitials (i.e., atoms occupying interstices between normal lattice sites), which are referred to as point defects. In extreme instances, the lattice damage is severe enough to transform the semiconductor target into an amorphous material.
As for silicon target material, ion implantation is the preferred technique for introducing p- and n-type dopant species, such as boron and phosphorus, respectively. The atomic collisions which occur during the implantation process create large numbers of the above-described point defects, including silicon interstitials produced by displacement of silicon atoms from their normal sites in the crystal lattice. Some of these defects agglomerate into small extended defects, such as rod-like interstitial defects that tend to orient along the (113) crystal direction. During the post-implantation thermal annealing step, the silicon interstitials (SiI) that are released from such agglomerates promote a rapid redistribution of the implanted dopant atoms, particularly p-type boron atoms. Redistribution occurs because silicon interstitials interact with substitutional boron atoms, thereby replacing the boron atoms and resulting in highly mobile boron interstitials (BI). The resulting redistribution of dopant caused by an increase in the effective diffusion rate of the dopants during the first few seconds of thermal annealing treatment is known as TED. In some instances, TED increases the diffusion rate by a factor of 10 to 100.
TED makes it difficult, if not impossible, to control implanted boron depth distribution in silicon targets. As a consequence, p-junction depth cannot be controlled by simply reducing the boron ion implantation energy (and thus the range of ion penetration into the silicon crystal lattice). Moreover, high temperature cycles, such as are encountered during annealing for activation of implanted dopant and lattice damage relaxation, can cause appreciable diffusion of the dopants, thereby significantly altering the dopant impurity concentration profiles, which effects are disadvantageously exacerbated by TED. Moreover, with the reduction of device sizes to the submicron range, there is a further need for close control of dopant diffusion. Techniques such as rapid thermal annealing (RTA) have been used to reduce undesirable diffusion of dopants during repair of implant damage. RTA refers to various techniques for heating the implanted target material for short periods of time, e.g., on the order of seconds.
However, even when RTA is utilized, TED has been observed during the post-implant annealing process. As indicated above, TED makes it difficult to control the implant profile. In the past, this was not a significant concern because the diffusion of dopants attributable to TED was considered to be within tolerable limits. But as technology now enables reduction of device sizes to below submicron dimensions, the effects of TED have become more pronounced and cannot be disregarded. For example, in MOS devices, lateral diffusion of the source and drain implants adversely affects the threshold-adjust implant at the gate region even though these regions are separated by, for example, several tenths of a micron. This results in an increase in gate threshold voltage VT which controls the conduction path between the source and drain as device size decreases (i.e., reverse short-channel effect). TED also causes impurity atoms to accumulate at the target surface in high concentration. This accumulation degrades channel mobility as it inhibits the movement of electrons between the source and the drain, thereby decreasing the speed of the resultant device. In addition, submicron-dimensioned devices require ultra-shallow junctions having depths of, e.g., less than 1000 xc3x85 to maintain low contact resistance. Vertical diffusion of dopants resulting from TED makes it difficult to achieve the shallow implant profiles necessary for such devices.
U.S. Pat. No. 5,670,391 issued Sep. 23, 1997 is directed to a process for fabricating a semiconductor device which includes a step for reducing the enhanced diffusion of dopant atoms that occurs during post-implant anneal. Prior to annealing the substrate wafer, the surface of the ion implanted region is etched to bring it closer to the level of the implant damage. The etch process is preferably controlled to prevent etching the surface significantly beyond the peak of the dopant concentration profile in order to avoid removing a significant portion of the implanted dopant. By bringing the implanted surface closer to the implant damage, the effects of TED during post-implant annealing are reduced.
U.S. Pat. No. 5,759,904 issued Jun. 2, 1998 discloses a method for suppressing TED of ion-implanted dopants in a semiconductor substrate comprising bombarding the substrate in a vacuum with a beam of bubble-forming ions at a first temperature, a first energy, and a first ion dose sufficient to form a dispersion of bubbles at a depth equivalent to a peak of lattice damage distribution in the substrate from implantation of dopant ions into the substrate in a vacuum at a second temperature, a second energy, and a second ion dose, the dispersion of bubbles being sufficient to reduce TED of the implanted dopant without inducing additional lattice damage in the substrate.
U.S. Pat. No. 5,559,050 issued Sep. 24, 1996 discloses a process for forming P-MOSFETS with enhanced anomalous narrow channel effect, comprising a step of performing an annealing step between a deep phosphorus implantation and growth of the gate oxide. The annealing step, performed at 800xc2x0 C. for 60 minutes, provides for rapid excess interstitial recombination at Si/SiO2 interfaces, thereby reducing TED of boron during the gate oxidation.
In view of the above, there exists a clear need for a rapid, simple, cost-effective method for eliminating or substantially reducing TED of implanted dopants during post-implantation thermal annealing of semiconductor materials, which method is fully compatible with the requirements of conventional process flow and manufacturing throughput, and provides increased product yield of submicron-dimensioned semiconductor devices.
An advantage of the present invention is an improved method for eliminating or substantially reducing TED of ion-implanted semiconductor materials.
Another advantage of the present invention is an improved method for eliminating or substantially reducing TED of ion-implanted dopants in silicon semiconductor substrates.
Still another advantage of the present invention is an improved ion-implanted monocrystalline silicon wafer substrate which experiences no or a substantially reduced amount of TED upon post-implantation thermal annealing processing.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the instant invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are achieved in part by a method of reducing TED of ion implanted dopant impurities within a semiconductor substrate, which method comprises introducing oxygen atoms into the substrate from an oxygen-containing layer overlying an implanted surface of the substrate for gettering interstitial defects within the substrate responsible for the TED.
In embodiments according to the present invention, the oxygen atoms introduced into the substrate comprise xe2x80x9cknocked-onxe2x80x9d atoms displaced from the oxygen-containing layer by ion bombardment; the ion bombardment preferably comprises implantation of ions into the oxygen-containing layer; and the thickness of the oxygen-containing layer and implantation species, dosage, and energy are selected for optimizing the displacement of oxygen atoms therefrom for supplying the substrate with oxygen atoms for gettering of interstitial defects.
In further embodiments according to the present invention, ion implantation into the oxygen-containing layer for displacement of oxygen atoms therefrom comprises implantation of n- or p-type dopant impurities typically selected from boron-, phosphorus-, arsenic-, and antimony-containing ions, which dopant impurities can be implanted into the oxygen-containing layer for displacing oxygen atoms therefrom simultaneously with implantation thereof into the semiconductor substrate for formation of a shallow junction therein. Alternatively, implantation of the dopant impurities into the oxygen-containing layer can be performed prior or subsequent to implantation of the dopant impurities into the semiconductor substrate for formation of a shallow junction therein.
In still further embodiments according to the present invention, ion implantation into the oxygen-containing layer for displacement of oxygen atoms therefrom comprises implantation of inert ions typically selected from germanium and silicon-containing ions; and the inert ions can be implanted in the oxygen-containing layer prior or subsequent to implantation of dopant impurities into the semiconductor substrate for formation of a shallow junction therein.
In preferred embodiments according to the present invention, the semiconductor substrate comprises a monocrystalline silicon wafer, the interstitial defects comprise interstitial silicon atoms, the oxygen-containing layer comprises a silicon oxide screen layer having a preselected thickness, and the silicon oxide screening layer is implanted with ions at a preselected dosage and energy to supply the silicon substrate with a sufficient quantity of xe2x80x9cknocked-onxe2x80x9d oxygen atoms displaced therefrom.
According to another aspect of the present invention, ion-implanted, doped monocrystalline silicon wafer substrates comprising a shallow junction depth below the wafer surface and having no or substantially reduced TED of the dopant species, are provided.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein only the preferred embodiment of the present invention is described, simply by way of illustration of the best mode contemplated for carrying out the method of the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.